Until I read Kerry’s response I thought they were eating the log on the left side instead of burying it.

Can they see people walking around looking out the right side of the tank and not the left side. Maybe they are seeing people and crowding in the corner and moving the gravel as a consequence? I know this was typical fish behavior when I had aquariums but my fish were smaller. Maybe these larger ones can move the gravel when they get excited from seeing people and thinking they will get fed. Also, maybe there is a slight current in the tank that helps move the gravel when they dislodge it.

Another reason might be that food, or waste, gravitates to that corner and they are sifting through the gravel and get it moving over time? Goldfish will eat about anything being from the carp family.

The yellow container under the fish tank is where I store partially used AA/AAA batteries, although at this stage we’ve pretty much changed to rechargeable batteries.

Those koi indeed love eating plants. If I put aquatic plants in the tank, they have them torn to shreds in the first day and no sign there was ever a plant after the third day.

It’s impressive they manage to move those artificial plants considering they have a wide heavy stone at the bottom. I must replace them the next time I shift the gravel back as they even manage to tear apart those artificial plants over time.

In Norway, there is an expression having the brain of a goldfish which was thought to have a memory a couple of seconds. Now the scientists did check that out and found that a goldfish could find its way through a maze to get food months even years after it was first introduced to the maze…

Process takes about 2 hours - Vacuuming the pebbles with a gravel cleaner to remove the gunk, scooping the pebbles to the right and refilling the tank with a set of hoses connected from the outdoor water butt to a heat exchanger and to the shower for the hot water supply.

They seem to move the pebbles in stages, i.e. pick up some and spit them out a little more to the left and repeat. They do indeed work through the night as even when I’m in bed, I can hear them bashing at the pebbles.

Have you got anything you could use to build a wall (front to back)across the tank? (Lego?) It would be an interesting to see if they are picking the stones up completely or just pushing them along the ground.

Regarding fish psychology…

In the wild many species of fish build nests in sand or gravel. Maybe they’re getting a bit broody?

They do pick up the pebbles in their mouths as every now and again when one swims to the top, it spits them out. I also had a few occasions where I go to take the lid off the black filter on the right to find there’s a few pebbles on top of the lid.

With my previous 4’ aquarium, I had a smaller filter pump with wider inlets. One cheeky koi often spat pebbles at the filter and occasionally managed to get a pebble or two inside. As you can imagine, this created a noisy racket with pebbles trapped inside the impeller and quite irritating when they do this in the middle of the night! I still could never figure out how the koi got the pebbles in there as any debris usually gets trapped in the filter’s sponge. The only clue was when the filter flow rate slowed down when it needed cleaning as a koi probably could have tried forcing pebbles into the output.

What is interesting is that they don’t cover the decoration boat in pebbles. There’s usually a few pebbles randomly dotted here and there on the boat, but what I could try is putting a jug worth on top and seeing whether they lift them out.

In our goldfish and aquatic frog tanks, we never had any unusual pebble shifting, let along pebbles stuck inside the filter pump.